1. Field of the Invention
The present invention relates to an electrophotographic resin-coated ferrite carrier having a small particle size, a spherical shape and a sharp particle size distribution, and yet providing low beads carry over and low variation with time of charge quantities and resistance in continuous printing when being used for a developer, a method for producing the same, and an electrophotographic developer using the resin-coated ferrite carrier.
2. Description of the Related Art
A two-component developer used in electrophotography is constituted of a toner and a carrier; the carrier is mixed and agitated with the toner in a developer box; the toner is given a desired charge; and the charged toner is carried to an electrostatic latent image on a photoreceptor whereby the developer is a carrier material to form a toner image. The carrier is, after having formed the toner image, held by a magnet and stays on a development roll, further returned to the developer box, again mixed and agitated with new toner particles, and repeatedly used in a certain period.
The two-component developer, different from a one-component developer, is one in which the carrier agitates the toner particles, imparts a desired charge, and has a function of transporting the toner, has good controllability in developer design, and is therefore widely used in the fields of full-color machines requiring high-quality images and high-speed machines requiring reliability and durability with respect to image sustainment.
In such a two-component electrophotographic developer, ferrite particles made of Cu—Zn ferrite, Ni—Zn ferrite or the like have been used as a carrier in place of an oxide-coated iron powder or a resin-coated iron powder in order to obtain high-quality images. Ferrite carriers formed of these ferrite particles commonly have many characteristics to form high-quality images, such as a high sphericity and adjustable magnetic properties, more advantageous than the conventional iron powder carrier. Furthermore, a resin-coated ferrite carrier coated with various resins using the ferrite particles as the carrier core has enhanced abrasion resistance and durability or the like. Also, the resin-coated ferrite carrier has adjustable specific volume resistance.
However, since the ferrite is ceramic, the ferrite after the ferritization reaction has high hardness. Adversely, the ferrite has the disadvantage that it is crushed by impact. When the particle size is particularly reduced, a space between the particles is also reduced, and the fusion of the particles is generated by high-temperature heating to complicate the sustainability of the spherical shape.
In recent years, in such a two-component electrophotographic developer, the high-speed and full-color of development performance have been strongly required, and under the requirements, the reduction in the particle size of the carrier or toner has been a subject in order to obtain high-quality images output.
Referring to the toner, there have been proposed various toners having a small particle size and a sharp particle size distribution by a polymerized toner technique or the like.
On the other hand, a formed magnetic brush is softened by reducing the particle size of the carrier, that is, by using the ferrite particles having a small particle size. Also, the specific surface area of the carrier is increased, and the quantity of the toner capable of being held is increased. As a result, larger effects have been expected for image quality such as image density, fog, toner scattering and gradation.
However, when the particle size of the ferrite carrier is reduced, unfortunately, it becomes difficult to sustain the spherical shape of the above-described ferrite particles. As described above, various resins are coated on the surface of the carrier core (ferrite particles) in order to enhance the abrasion resistance and the durability. However, when the shape of each of the ferrite particles is impaired, coating unevenness is generated in resin coating, or the exposed part of the core material is generated. Therefore, the carrier performance is not sufficient to attain a higher image quality and a longer life (higher durability) required of the developer.
Also, when the particles are ground in the grinding step after baking in the producing step of the ferrite particles, and the fused particles are ground by strong impact, the particles are crushed, and irregular particles are mixed. It is difficult to remove the irregular particles, which cause beads carry over. If the resin is coated as it is in the next step, a uniform film cannot be formed on each of the irregular particles, and the irregular particles hinder flowability, causing adverse effects to the image quality.
Although the fusion between the particles can be prevented by reducing a baking temperature in order to sustain the spherical shape, the carrier core becomes porous (porosity), and the resin is oozed into the inside in the resin coating step to the surface of the carrier core, and the resin is apt to become the variation factor of the carrier performance.
Thus, techniques for producing the ferrite particles having a spherical shape, a uniform surface nature and a small particle size have been insufficient. There have been made various efforts for providing the ferrite carrier having a spherical shape, a uniform surface nature and a small particle size in order to attain a higher image quality and a longer life when used as a two-component developer together with a toner.
Japanese Patent Laid-Open No. 7-98521 discloses an electrophotographic carrier having a particle size distribution specified by a 50% average particle size (D50) of 15 to 45 μm and a fixed ratio of specific surface areas determined by different measuring methods.
Also, Japanese Patent Laid-Open No. 2001-117285 discloses a carrier for electrostatic image development comprising core particles (carrier core) which have a volume average particle size of 25 to 50 μm and certain ranges of volume resistance and shape factor, and a coating layer formed on the surface of each of the core particles and containing conductive particles.
Japanese Patent Laid-Open No. 8-292607 discloses a two-component developer comprising carrier core particles having a coating layer formed on the surface thereof and made of a resin material, and specified shape factor of the carrier core particles and carrier particles after resin coating, the former shape factor being larger than the latter shape factor.
Japanese Patent Laid-Open No. 9-197722 discloses a carrier for electrostatic image developer which has saturation magnetization of 50 to 70 Am2/kg, an average particle size of 30 to 40 μm, a weight ratio of particles having an average particle size of 22 μm or less being 2.0 to 17.0% by weight, and a coating layer formed on core particles (carrier core) specified by a shape factor.
Although the above-described Patent Documents reduce the particle size of the ferrite core material, specify the shape factor and the specific surface area or the like, and mainly obtain the spherical ferrite core material, a carrier core or a resin-coated ferrite carrier having a small particle size, a high spherical degree, a high surface evenness and a sharp particle size distribution, and a method for producing the same have not been obtained.
Also, when a resin-coated ferrite carrier is used as a developer together with a toner, the developer is required to have small variations with time in charge quantity and resistance in continuous printing. However, a carrier core or resin-coated ferrite carrier for satisfying both of these requirements, and a method for producing the same have not been obtained.